Fine denier fiber has advantages of being fine and soft, having silky hand-feeling, delicate color and strong fabric coverage as compared with ordinary fibers. Imitation silk made from fine denier fiber has soft hand feeling and gorgeous appearance, and is an excellent material for making top-grade full dresses, top grade down coats, shirts and underlinens. Therefore, the study on fine denier fiber is of great economic interest.
At present, fine denier fiber has been one of the leading products of “new synthetic fibers”. Efforts have been made to study terylene fine denier fiber, polypropylene fine denier fiber, and acrylic fine denier fiber have been greatly developed, and the productions thereof are industrialized
Nylon (polyamide, PA) is more hydrophilic, moisture-absorbing, gas permeable and has better affinity to skin, as compared with terylene fiber and polypropylene fiber. However, there are great deal of hydrogen bonds in the molecular structure of Nylon (polyamide, PA), such that it has high crystallinity, is crystallizable easily, and has low tensile deformation level during spinning. Therefore, it is difficult to produce fine denier polyamide fiber by direct melt-spinning. Fine denier Nylon fiber has been produced by composite spinning processes, and the obtained fibers have good properties. For example, Chinese patent CN101333693B provides a method for producing a fine denier polyamide staple fiber, by which a fine denier Nylon fiber having mono-filament linear density of 0.05-0.3 dtex may be obtained, with spinning speed of about 300 to 800 m/min. However, the composite spinning processes have following disadvantages. Firstly, the spun bi-component fiber should be retreated by an alkali solution or an organic solvent to obtain the fine denier fiber, thus the process is complicated as compared with direct melt-spinning; secondly, it is difficult and costly to recover polymer dissolved by the alkali solution or the organic solvent during the re-treatment, thus the production cost increases greatly and it is possible to cause environmental problem during emission; thirdly, a specific spinning assembly is needed during spinning, thus the process is complicated.
Currently there are few methods for producing fine denier fiber via direct melt-spinning. Chinese patent CN 101139749B discloses a method for producing a fine denier Nylon fiber having mono-filament linear density less than 1 dtex by modifying Nylon with metallic compound followed with direct melt-spinning. However, though this method can obtain a fine denier Nylon fiber having mono-filament linear density less than 1 dtex, it has following deficiencies. Firstly, the method achieves its purpose by incorporating metallic compound masterbatch into Nylon matrix. As the amount of the masterbatch increases, the metallic compound will aggregate to conglomeration, whereby the spinneret is plugged, and fibers can not flow out smoothly, thus the filament fluffiness and fiber breakage are increased, and the strength of the obtained fiber will be dropped greatly (Chaowei HAO, process for producing fine denier porous Nylon 6 elastic fiber, synthetic fiber industry, 2011, 34 (6): 41-43). Secondly, because of the conglomeration of the metallic compounds in the polyamide melt, the incorporated amount of the metallic compound masterbatch and the spinning speed are restricted, which further influences the modifying effect on polyamide. For example, the article “process for producing fine denier porous Nylon 6 elastic fiber” (Chaowei HAO, etc., China Synthetic Fiber Industry, 2011, 34(6), p.41-43) points out that the fiber properties will be best when mass percentage of rare earth in the fiber is 0.08% and the spinning speed is 3800 m/min. A related document (Chunhua GUO, New Fine Denier Nylon Fiber Attract Much Attention, Textile and Apparel Weekly, 2010, 6:32) reports that the mono-filament linear density of the fine denier Nylon fiber mass produced by this technology only reaches to 0.6dtex, thus this process can only obtain the fine denier Nylon fiber having linear density of 0.6dtex-1 dtex.
Therefore, all these mentioned technologies can not reasonably obtain fine denier Nylon fiber. There is a need to find feasible and effective new means for producing fine denier Nylon fiber.